The present invention relates to an arrangement and a method to reduce the build-up formed on the grate of a fluidized-bed furnace in the roasting of fine-grained material such a concentrate.
1. Field of the Invention
The concentrate is fed into the roaster from the wall of the furnace, and oxygen-containing gas is fed via gas nozzles under the grate in the bottom of the furnace in order to fluidize the concentrate and oxidize it during fluidization. Below the concentrate feed point, or feed grate, the oxygen content of the gas to be fed is raised compared with gas fed elsewhere with additional gas jets situated higher in the feed grate than the other jets. The extra jets of the feed grate are connected to their own gas distribution unit.
2. Description of the Related Art
The roasting of fine-grained material such as zinc concentrate usually takes place using the fluidized bed method. The material to be roasted is fed into the roasting furnace via feed units in the wall of the furnace above the fluidized bed. On the bottom of the furnace there is a grate, via which oxygen-containing gas is fed in order to fluidize the concentrate. There are usually in the order of 100 gas jets/m2 under the grate. As the concentrate becomes fluidized, the height of the feed bed rises to about half that of the fixed material bed.
The concentrate in the fluidized bed is oxidized (burnt) to a calcine by the effect of the oxygen-containing gas fed via the grate, e.g. zinc sulfide concentrate is roasted into zinc oxide. In zinc concentrate roasting the temperature to be used is in the region of 900-1050xc2x0 C. The calcine is partially removed from the furnace through the overflow aperture, and partially it travels with the gases to the waste heat boiler and from there on to the cyclone and electrostatic precipitators, where the calcine is recovered. In general the overflow aperture is located on the opposite side of the furnace to the feed units. The calcine removed from the furnace is cooled and ground finely for leaching.
For good roasting it is important to control the bed i.e. the bed should be good and the fluidizing controlled. Combustion should be as complete as possible, i.e. the sulfides should be oxidized into oxides. The calcine should also come out of the furnace well. The particle size of the calcine is known to be affected by the chemical composition and mineralogy of the concentrate as well as by the temperature of the roasting gas.
In the technique currently in use the roaster concentrate feed is regulated according to the temperature of the bed using for example fuzzy logic. Thus there is a danger that the amount of oxygen in the roasting gas may drop too low i.e. that the amount of oxygen is insufficient to roast the concentrate. At the same time the back pressure of the bed may fall too low.
It is known from balance calculations and balance diagrams in the literature that copper and iron together form oxysulfides, which are molten at roasting temperatures and even at lower temperatures. Similarly, zinc and lead as well as iron and lead together form sulfides molten at low temperatures. This kind of appearance of sulfides is possible and the likelihood grows if the amount of oxygen in the bed is smaller than that normally required to oxidize the concentrate.
During fluidized bed roasting agglomeration of the product normally occurs, i.e. the calcine is clearly coarser than the concentrate feed. The above-mentioned formation of molten sulfides however increases agglomeration to a disturbing degree, in that the larger agglomerates with their sulfide nuclei remain moving around the grate. The agglomerates cause build-ups on the grate and with time block the gas jets under the grate. It has been noticed in zinc roasters that build-ups containing impure components are formed in the furnace particularly in the section of the grate under the concentrate feed units.
In the prior art, for example in DE application publication 42 11 646, a gas feed arrangement for a fluidized bed has been described. It was stated to be a problem that the material to be fluidized tends to settle back into the furnace at the edges of the furnace and particularly back to the solids feed point, such as for instance a build-up tending to form on the furnace grate under the feed point of material returning to the cycle. In order to avoid build-ups, the gas jets, particularly in that part of the grate where the bed material is returned, and at the edges of the furnace, are to be raised higher than the jets in the central part (longer nozzle arm head). The purpose is that the nozzles are at the same distance from the bottom or the solids at all points in the furnace. Some of the jets in the furnace may be raised higher than others, also in the central part of the grate, in order to prevent build-ups. The jets blow the gas to the side or down. All the jets are connected to the same gas distribution unit i.e. the gas feed is uniform.
When a great deal of impure, highly reactive concentrate is fed to a roasting furnace, an oxygen deficit is caused in the immediate vicinity of the feed unit preventing the oxidation of the concentrates to oxides, i.e. the actual purpose of roasting. As a result, a molten sulfidic material of low temperatures is formed, which agglomerates. The larger agglomerates sink to the grate, remain there rotating and combine to form a layer of build-up, which blocks the gas jets.